Pachycephalosaurs are a small group of dinosaurs which are best known for the thick dome-shaped skulls.  All primitive species are known from Asia.  Since they appear to have evolved in Asia, as did the early ceratopsians to whom the pachycephalosaurs are most closely related, Asia seems to be the birthplace of both groups.  Of the more than a dozen species, all but one belongs to the Late Cretaceous.  Some species are more flat-headed while in others the dome is larger.  Separate lineages might have increased dome size separately.  (Lambert, 1990, Fastovsky, 1996). 

    Pachycephalosaurs had different kinds of teeth with canine-like teeth in the premaxillary bone of the upper jaw (Maryanska, 1972).  There was sexual dimorphism in the shape of the dome. 

pachycephalosaurus GENDER DIFF DOME

     This dome was solid bone-- although the skull became larger, there was no increase in the size of the braincase.  The skull of Pachycephalosaurus is 65 cm long and the dome is 20 cm thick.  The upper temporal fenestra was closed by this bony dome.  Was this dome used for head butting?  The bone of the dome was dense and aligned for stresses coming in that direction.  Scars on the domes indicate that it was involved in some violent acts.  The back was very strong with tight junctions between vertebrae and with strong neck muscles.  The occiput (where skull meets vertebral column) was redesigned so that it lined up with the back to better deal with stress from head on collisions. The sexual dimorphism in the thickness of the dome indicate that one of its uses was for intraspecific combat (presumably intermale combat).

     Pachycephalosaurs were low browsers and their wide rib cage suggests that they performed some fermentation of plant material.  Although their brain size was average for dinosaurs, the optic and olfactory nerves was enlarged.  Their optic nerves are about the largest seen in dinosaurs and their orbits (eye sockets) are also large (Lambert, 1990, Fastovsky, 1996).


     The Homalocephalidae, “even heads”, may not represent a clade, but rather 4 genera becoming progressively closer to higher pachycephalosaurs of the family Pachycephalosauridae.  Micropachycephalosaurus was a cat-sized species which currently represents the dinosaur with the longest name.  Stegoceras is intermediate between flat-headed and domed pachycephalosaurs (Sullivan, 2003). The Pachycephalosauridae developed large crests and were not fast runners.


homalocephale pachy skulls